Older brother is a kilt kop

[Wandering Spark]

How do I deal with him? He always makes fun of kilts and always tells me that wearing it over my trousers isn’t “how it’s meant to be worn” I don’t care and I don’t care for his dislike of Scottish culture either, I’d appreciate some words to arm myself with so I can wear my kilt around him.

To address the original question: use your brother's dislike of Scotland against him. ex. 'You hate Scottish culture anyway, why do you care if I'm not wearing my kilt "correctly"?'

As a side note, none of the highland wear is traditional as such, it’s all Victorian inventions because kilts were en vogue, none of it is trad, unless you want to wear a great kilt (without a shirt underneath either as those are a modern invention)

While the modern, tailored shirt is a relatively recent development, loose-fitting shirts were part of Highland attire long before the great kilt was invented.

The oldest surviving Highland shirt:
Rogart shirt Bog bodies 1995 small.jpg
found on a bog body in Rogart, Sutherland, believed to date to the 13th-14th century.

Highlanders in the 16th c. wore voluminous linen shirts which the wealthy dyed yellow with saffron.
In 1521, John Major described Highland attire as follows:
"From the middle of the thigh to the foot they have no covering for the leg, clothing themselves with a mantle instead of an upper garment and a shirt dyed with saffron [. . .] The common people of the Highland rush into battle having their body clothed with a linen garment manifoldly sewed and painted or daubed with pitch, with a covering of deerskin."

In 1556, Jean de Beaugué described the Highland clothing as follows:
"They wear no clothes except their dyed shirts and a sort of light woolen rug of several colours."

In 1578, Bishop Lesley described these Scottish shirts in more detail:
"They also made of linen very large shirts, with numerous folds and wide sleeves, which flowed abroad loosely to their knees. These, the rich coloured with saffron and others smeared with some grease to preserve them longer clean among the toils and exercises of a camp, which they held it of the highest consequence to practice continually. In the manufacture of these, ornament and a certain attention to taste were not altogether neglected, and they joined the different parts of their shirts very neatly with silk thread, chiefly of a red or green colour."

The earliest evidence for the great kilt on the other hand, comes from Lughaidh O'Clery's 1594 description of Scottish attire:
"[. . .] their exterior dress was mottled cloaks to the calf of the leg with ties and fastenings. Their girdles were over the loins outside the cloaks."
(translations from McClintock 1943)

The earliest known image of a great kilt is from the album of Hieronymus Tielsch, created circa 1603–1616:
Tielsch c1603 cropped .jpg
He is wearing a shirt under his great kilt.

There is evidence from the later 17th and early 18th c. of men wearing great kilts without shirts, like this illustration of a Highland man from John Speed's map of Scotland published in 1662.
Speeds map highland man 1662 200.jpg

Also in Captain Edward Burt's Letters from a Gentleman in the North of Scotland which were written in the 1730s:
"The common habit of the ordinary Highlanders is far from being acceptable to the eye: with them a small part of the plaid, [. . .] is set in folds and girt round the waist, to make of it a short petticoat that reaches half way down the thigh, and the rest is brought over the shoulders, and then fastened before, below the neck, often with a fork, and sometimes with a bodkin or sharpened piece of stick, so that they make pretty nearly the appearance of the poor women in London when they bring their gowns over their heads to shelter them from the rain. In this way of wearing the plaid, they have sometimes nothing else to cover them, and are often barefoot;"

However, this wearing a great kilt without anything under it appears to be the result of poverty rather than an intentional dress tradition, because in the same letter Burt says that the dress of Highland gentlemen includes: "a bonnet made of thrum without a brim, a short coat, a waistcoat, longer by five or six inches, short stockings, and brogues, or pumps without heels."

In summary, a great kilt without a shirt under it could be considered 'traditional Highland wear,' but a shirt without a great kilt over it could also be considered 'traditional Highland wear,' and shirt without a kilt is actually the older version. What is Highland attire has been changing for centuries.

While I agree with the general sentiment that you can't strictly define 'traditional Highland attire', I think it's a bit unfair to to write off modern 'traditional Highland wear' as not traditional because it's 'Victorian invention'. Technically, the great kilt is just a Jacobean invention. Yes, the Ban, the Gaelic Revival, and Victorian romanticization caused large changes in Highland dress, but even if those things had never happened, I guarantee some changes would have happened in Highland dress during the 19th c. The English certainly don't dress the same way as they did in 1745.

*getting off my soapbox now*

You are welcome to wear your kilt however you want. I don't care how you wear it, as long as you don't show up to a Jacobite reenactment event claiming your kilt-over-trousers outfit is authentic 18th c. attire. If you want to learn about the history of Highland Dress, I recommend reading The costume of Scotland and History of Highland Dress by John Telfer Dunbar.

Sources:
Dunbar, John T. (1964). History of Highland Dress. Dufour Editions, Philadelphia.

McClintock, H. F. (1943). Old Irish and Highland Dress. Dundalgan Press, Dundalk.

Turner, R. C. and Scaife, R. G. (1995). Bog Bodies: New Discoveries and New Perspectives. British Museum Press, London.